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Origins of Magnetite Nanocrystals in Martian Meteorite ALH84001

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Origins of Magnetite Nanocrystals in Martian Meteorite ALH84001 Author's personal copy Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 6631–6677 www.elsevier.com/locate/gca Origins of magnetite nanocrystals in Martian meteorite ALH84001 K.L. Thomas-Keprta a,*, S.J. Clemett a,*, D.S. McKay b, E.K. Gibson b, S.J. Wentworth a a ESCG at NASA/Johnson Space Center, Houston, TX 77058, USA b KR, ARES, NASA/Johnson Space Center, Houston, TX 77058, USA Received 20 July 2008; accepted in revised form 18 May 2009; available online 16 June 2009 Abstract The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe3O4) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of mag- netite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the com- positional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites pres- ent in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates. Ó 2009 Published by Elsevier Ltd. 1. INTRODUCTION The majority of these carbonates appear as circular or ellip- tical features, which range from 10 to 300 lm along the The Allan Hills 84001 (ALH84001) meteorite is a coarse major axis (i.e., eccentricity >0.5). When viewed optically grained orthopyroxenite that solidified from a melt within an exposed fracture surface they range in color from 4.51 Ga ago (Nyquist et al., 2001). Approximately gold to burnt orange in their centers and are typically sur- 600 Ma later (Borg et al., 1999), secondary carbonates were rounded by a thin black–white–black rim (Fig. 1A). The deposited by low temperature hydrothermal processes in three dimensional morphology of these carbonates can best pre-existing fractures and fissures within the groundmass. be described as an inverted conic frustum in which the base of the conic lies flush with the orthopyroxene (OPX) surface and the apex lies under the exposed surface (Fig. 1B). For * Corresponding authors. Tel.: +1 281 483 5029. reasons of brevity we will subsequently user the simpler, al- E-mail addresses: [email protected] (K.L. beit less accurate, description of ‘disk’ when referring to Thomas-Keprta), [email protected] (S.J. Clemett). these carbonates. 0016-7037/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.gca.2009.05.064 Author's personal copy 6632 K.L. Thomas-Keprta et al. / Geochimica et Cosmochimica Acta 73 (2009) 6631–6677 Fig. 1A. Optical images of ALH84001 disk-like carbonates. These disks precipitated in fractures produced during shock impacts while the meteorite was on Mars. The majority of these carbonates appear as circular or elliptical features which range from 10 to 300 lm along the major axis. Visually they have colors that vary from gold to burnt orange in their centers which are typically surrounded by a thin black– white–black rim. Some carbonates occur as single entities while others are spatially associated and occur in groups of 10s of carbonates (image at far right). The black rims typically range from 5–10 lm in thickness and are composed primarily of nanophase magnetite embedded in a matrix of Mg-bearing sideritic carbonate while the white bands, typically 10–15 lm thick, are composed of nearly pure magnesite with minor Ca and nanophase magnetite. thin rim. Embedded within these carbonates are discrete nanocrystal magnetites (Fe3O4) that exhibit super-para- magnetic to single-domain properties (Muxworthy and Wil- liams, 2008). While the largest population of these magnetites occurs within the rim zone of the carbonate disks, e.g., McKay et al. (1996) and Thomas-Keprta et al. (2000a), a small but significant fraction is also found within the inner and outer core zones. When the carbonate disks were initially characterized, they were interpreted as products of high temperature pro- cesses (T > 573 K) (Harvey and McSween, 1996; Mit- tlefehldt, 1994a; Mittlefehldt, 1994b; Scott et al., 1997). Subsequently, isotopic and chemical data has forced a revi- sion in this interpretation and it is now generally accepted they were deposited by low temperature (T < 573 K) hydro- Fig. 1B. Illustration of an inverted conic frustum, that is, the thermal processes (Eiler et al., 2002; Kent et al., 2001; Kirs- polygon which remains after a cone is cut by a plane parallel to the chvink et al., 1997; McSween and Harvey, 1998; Romanek base with the apical part removed and the remaining polygon et al., 1994; Saxton et al., 1998; Treiman, 1997; Valley et al., inverted. While single ALH84001 carbonates have been loosely described as rosettes, globules, concretions, or pancakes, they are 1997; Warren, 1998). While the consensus of opinion on the more accurately described as being shaped like an inverted conical temperature of formation of the carbonates has converged, frustum. that of their subsequent thermal evolution has not because of the differing hypotheses invoked to explain the presence of the nanocrystal magnetites present within the carbonate. Chemically the disk carbonates have a complex mixed To first order, these magnetites were either already present cation composition with an empirical formula (FexMgy- when the carbonates formed and so became embedded dur- CazMn[1ÀxÀyÀz])CO3 where x + y + z = 1. The values of ing carbonate deposition, or they were formed a posteriori x, y, and z vary with radial distance in such a way that by partial thermal decomposition of the pre-existing car- the carbonate disks can be envisioned as being composed bonate. In the latter case, the heating of the meteorite is of three concentric annular zones; starting from the center envisioned to have occurred through the adiabatic propaga- there is an inner central and outer core surrounded by a tion of one or more impact shock events. Author's personal copy Origins of Magnetite in ALH84001 6633 ALH84001 shows evidence of having undergone multi- 1996; Thomas-Keprta et al., 2000a; Thomas-Keprta et al., ple impact events, the last resulting in its ejection from 2001; Thomas-Keprta et al., 2002). Mars, e.g., Treiman (1998). In general, the peak shock pres- sures experienced by Martian meteorites are thought to 2. METHODOLOGY range from less than 14 gigapascals (GPa) up to 55 GPa (Fritz et al., 2004; Fritz et al., 2003; Langenhorst et al., 2.1. ALH84001 carbonate disks 2000; Sto¨ffler, 2000; Treiman, 2003; van der Bogert et al., 1999). These estimates are based on the conversion of pla- Table 1 lists the ALH84001 rock splits used in the re- gioclase to either a diaplectic ‘maskelynite’ (14–45 GPa) search presented herein. Prior to any extraction, fracture or melt glass (>45 GPa) with a resulting increase in the surfaces containing carbonate disks were first characterized refractive index and/or Raman band broadening, compared by optical and electron microscopy. This involved optical to unshocked plagioclase (Fritz et al., 2004). Specifically, imaging under both bright and dark-field illumination con- for ALH4001, estimated values for peak shock pressure ditions. Given the topographic variations present within a have evolved over time – 50–60 GPa (Langenhorst given fracture surface, a long working distance Nikon et al., 2000); <40 GPa (van der Bogert et al., 1999); 35– Eclipse ME600 microscope was used for this task. Due to 40 GPa (Sto¨ffler, 2000); 35.7 ± 4.5 GPa (Fritz et al., the low numerical apertures associated with long working 2003); >35–40 GPa (Fritz et al., 2004) – with the best cur- distance microscope objectives, the depth-of-field was rent estimate at 32 ± 1 GPa (Fritz et al., 2005). The extent to which impact shocks to ALH84001 lead to thermal excursions that could have partially decomposed Table 1 the ALH84001 carbonate disks remains ambiguous (note, Rock splits of Martian meteorite ALH84001. in this context any impacts occurring in the 600 Ma prior Split Parent to carbonate formation have no bearing). Results from sim- 157 21 ulations of shockwave propagation through composition- 158 40 ally and texturally complex matrices are computationally 160 2 difficult and sensitive to initial starting conditions, but do 161 17 indicate that shock heating would have been highly hetero- 162 41 geneous at all size scales. Using a Hugoniot equation-of- 163 69 state experimentally derived from Stillwater pyroxenite, 164 76 165 103 Twin Sister dunite (Sto¨ffler, 1982), and a gabbroic rock 166 147 (Trunin et al., 2001), the post-shock temperature elevation 174 39 for ALH84001 associated with a 32 GPa shock event lies 175 39 in the range of 373–383 K (Fritz et al., 2005).
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